1. Field of the Invention
The present invention relates generally to a device and method of continuous outer-loop power control in a discontinuous transmission (DTX) mode for a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a device and method for implementing an outer-loop power control in a non-frame data transmission period as well as a frame data transmission period.
2. Description of the Related Art
For packet transmission in the American IMT-2000 standard, IS-95C, P1, P2, and P3 options are used. The P1, P2, and P3 options utilize a fundamental traffic channel and a supplemental channel, a fundamental traffic channel and a dedicated control channel (DCCH), and a fundamental traffic channel, a DCCH, and a supplemental channel, respectively. Control information about a packet and a signaling message are transmitted on the fundamental traffic channel and the DCCH and packet data is transmitted on the supplemental channel. The control information and the signaling message do not occur all the time. When no control information and signaling message exist, the fundamental traffic channel transmits null traffic, whereas the DCCH transmits power control bits (PCBs) on a forward link and pilot symbols and PCBs on a reverse link. The mode of the DCCH is termed a DTX mode during which only null frames are transmitted when there is no transmission frame data. The fundamental traffic channel and dedicated control channel (DCCH) are dedicated channels. In other words, it is also a dedicated channel that the channel is assigned to a specific user in traffic period.
For power control, an outer-loop power control and a closed-loop power control are concurrently performed in the DTX mode. The closed-loop power control refers to controlling power for each power control group (PCG), using a threshold determined for each frame. On the other hand, the outer-loop power control scheme changes the threshold set for the closed-loop power control depending on the presence or absence of frame errors. Specifically, the threshold is increased or decreased by a predetermined level according to whether a frame has errors or not. Then, a closed-loop power controller implements a closed-loop power control using the changed threshold. In the case that the outer-loop power control and the closed-loop power control are employed together, the closed-loop power control is implemented using a threshold determined by the outer-loop power control upon presence of a frame and an existing threshold upon absence of a frame, in a DTX mode.
A description of power control in a DTX mode for a communication system employing both the outer-loop power control scheme and closed-loop power control scheme is provided below.
FIG. 1A is a block diagram of a forward link transmitter in a general CDMA mobile communication system. Referring to FIG. 1, insertion of PCBs in a DTX mode will be described.
In FIG. 1, a control message buffer 111 is a memory for temporarily storing a control message to be transmitted on a DCCH. The capacity of the control message buffer 111 can be set to one or more frames. The control message buffer 111 interfaces a control message between a higher-layer processor and a MODEM controller 113. The higher-layer processor stores a control message with header information for identifying a frame according to a message type in the control message buffer 111 and sets a flag to indicate the storage. The MODEM controller 113 reads the control message from the control message buffer 111 and then clears a flag to indicate the reading. By the operations, the higher-layer processor and the MODEM controller 113 prevent over-writing and over-reading.
After reading the control message from the control message buffer 111, the MODEM controller 113 determines a message type by analyzing the header of the control message, and outputs a payload to be transmitted on a DCCH according to the message type and a corresponding control signal. The output control message is variable in duration, that is, 5 or 20 ms according to the analysis result. In the following description, no distinction is made between a 5 ms-control message and a 20 ms-control message. The MODEM controller 113 determines whether there is a control message to transmit and controls transmission of the DCCH. That is, the MODEM controller 113 generates a first gain control signal upon presence of a control message to be transmitted and a second gain control signal for blocking signal transmission on the DCCH upon absence of a control message. The gain control signals are signals for controlling the transmission power of the DCCH. While the multiplier 125 is located at the frontal end of a spreader, the same effect can be produced even if it is at the rear end of the spreader.
A CRC (Cyclic Redundancy Check) generator 115 adds a CRC to the control message received from the MODEM controller 113 to allow a receiver to determine the quality of a frame, that is, the presence or absence of a frame. The CRC generator 115 outputs a control message with the CRC under the control of the MODEM controller 113. A 40-bit control message with a 16-bit CRC is generated for a 5 ms-frame, and a 184-bit control message with a 12-bit CRC for. a 20 ms-frame.
A tail bit encoder 117 analyzes the output of the CRC generator 115 and adds corresponding tail bits to the output of the CRC generator 115, for terminating an error correction code. Here, the tail bit encoder 117 generates 8 tail bits.
An encoder 119 encodes the output of the tail bit encoder 117 at a code rate of ⅓. The encoder 119 can be a convolutional encoder or a turbo encoder. An interleaver 121 permutes the bit sequence of encoded symbols received from the encoder 119 in frame units to protect the data from burst errors.
The CRC generator 115, the tail bit encoder 117, the encoder 119, and the interleaver 121 form a control message generator 150 for generating a control message and transmitting it on a physical channel. While the control message generator 150 processes a control message for a frame in FIG. 1A, it can be contemplated that the MODEM controller 113 selects a control message generator corresponding to the length of a frame to transmit among as many control message generators as the frame lengths of control messages transmitted on the DCCH. In this case, each control message generator should be provided with a CRC generator, a tail bit encoder, an encoder, and an interleaver according to the frame length of a control message processed in the control message generator.
A signal mapper 123 maps 1s and 0s of the interleaved symbols to xe2x88x921s and 1s, respectively. A gain multiplier 125 performs a DTX mode function by establishing a path for transmitting the DCCH control message or blocking the path depending on which gain control message is received from the MODEM controller 113.
A PCB puncturer 129 inserts a PCB into a signal received from the multiplier 125. A serial-to-parallel converter (SPC) 127 multiplexes control message symbols received from the PCB puncturer 129 and distributes the multiplexed symbols to carrier spreaders. Here, three carriers are used by way of example. For the three carriers, six channels are produced from three carrier frequencies and two phases (I and Q channels) of each carrier. The PCB can be used for controlling reverse link power of a mobile station.
FIG. 1B is a block diagram of a spreader for spreading symbols received from the PCB puncturer 129. A forward link transmitter includes as many spreaders as carriers. For example, three spreaders exist in the forward link transmitter shown in FIG. 1A.
Referring to FIG. 1B, an orthogonal code generator 135 generates a DCCH orthogonal code which can be a Walsh code or a quasi-orthogonal code. Multipliers 131 and 133 multiply I- and Q-channel signals of the forward DCCH control message by the orthogonal code, for orthogonal spreading.
A modulator 137 PN-spreads the orthogonally spread I- and Q-channel signals received from the multipliers 131 and 133 with PN codes PNi and PNq received from a PN sequence generator (not shown). A complex multiplier can be used as the modulator 137.
The MODEM controller 113 controls transmission of the DCCH in a DTX mode. That is, the MODEM controller 113 performs a DTX mode control according to the capacities of signals for data service and MAC-related messages (Medium Access Control) communicated on the DCCH, to thereby use channel capacity efficiently. Since voice traffic and signal traffic are multiplexed in IS-95, both a voice channel and a signaling channel are typically opened all the time for data service. However, the DCCH operates in the DTX mode and thus need not be opened for a control signal. If no signaling information is to be transmitted, a DTX gain controller like the MODEM controller 113 reduces transmission power for efficient use of radio resources.
The above embodiment is about a 3x system using a multi-carrier and can be applied to a transmitter in a 1x or 3x DS system (Direct Sequence). Thus, a description of the 1x or 3x DS system will be omitted.
FIG. 2 is a block diagram of a reverse link transmitter which operates in a DTX mode for a conventional CDMA mobile communication system. As shown in FIG. 2, the reverse link transmitter is similar to the forward link transmitter in structure. Therefore, a description of the same components will be omitted.
An orthogonal spreader 207 generates a Walsh code. A first multiplier 209 multiplies a transmission signal received from a signal mapper 205 by the Walsh code received from the orthogonal spreader 207, for orthogonal spreading. A gain multiplier 221 outputs a gain value for a message, or outputs no data upon receipt via gain controller 219 of a gain control signal 0 from a MODEM controller 203 if there is no transmission message and data upon receipt of a gain control signal 1 from the MODEM controller 203 if a transmission message exists. A summing device 223 forms a DCCH signal by summing the transmission signal received from the gain multiplier 221 and a pilot/PCB channel signal. A PN spreader 225 complex-PN-spreads the DCCH signal.
A description of the structures and operations of forward and reverse link receivers for performing an outer-loop power control and a closed-loop power control using a reverse pilot channel and a PCB received on a forward DCCH follows with reference to FIGS. 3 and 4, respectively.
FIG. 3 is a block diagram of a reverse link receiver in a DTX mode for a conventional CDMA mobile communication system.
Referring to FIG. 3, a first despreader 301 is a PN despreader for PN-despreading a received signal. A second despreader 303 is a DCCH Walsh despreader for despreading a DCCH signal included in the PN-despread signal received from the first despreader 301 with a Walsh code. A channel estimator 305 detects a fading component using a pilot channel included in the PN-despread signal received from the first despreader 301. A third despreader 307 is a pilot channel Walsh despreader for despreading the pilot channel signal included in the PN-despread signal received from the first despreader 301 with a Walsh code.
A multiplier 314 multiplies the complex conjugate of the fading component received from the channel estimator 305 by the DCCH signal received from the second despreader 303 in symbol units, for error compensation. A PCB extractor 317 extracts a PCB from the error-compensated DCCH signal received from the multiplier 314. A bit energy measurer 309 measures bit energy Eb from the PCB received from the PCB extractor 317 and the fading component received from the channel estimator 305. A noise measurer 311 measures noise energy Nt from the symbol value of the pilot channel received from the third despreader 307 and the fading component from the channel estimator 305. An SNR calculator 313 calculates an SNR from the noise energy Nt and the bit energy Eb. For details of an Eb and Nt measuring method, see xe2x80x9cForward Link Closed Loop Power Control Method for CDMA 2000-(Rev. 1)xe2x80x9d, Stein Lundby, Contribution to TR45.5.3.1./98.12.08.28.
A decoder 319 decodes the output of the PCB extractor 317 and a CRC error detector 321 performs a CRC error check on the decoded signal received from the decoder 319. The output of the CRC error detector 321 is True (1) or False (0). Since the DCCH channel is transmitted in the DTX mode, the receiver calculates a CRC from a frame if the frame has transmission data to determine whether a frame error has occurred. For details of a method of determining whether a DCCH has frame data or not in a DTX mode, see Korea Application No. 98-04498. A data detector 323 receives frame data and a CRC error check result from the CRC error detector 321 and generates an on/off control signal to a MODEM controller 325. The MODEM controller 325 is activated by the on/off control signal to detect a control message from the decoded data received from the decoder 319 and to store the control message in a control message buffer 327.
If the receiver performs a closed-loop power control alone, a closed-loop power controller 315 compares the SNR of each PCB received from the SNR calculator 313 with a fixed threshold and controls power according to the result of the comparison. If the receiver performs a closed-loop power control and an outer-loop power control together, an outer-loop power controller 329 is further provided to the receiver. The outer-loop power controller 329 determines a threshold and then the closed-loop power controller 315 performs a closed-loop power control using the threshold. The outer-loop power controller 329 is activated upon receipt of a frame existence flag from the data detector 323 and determines the threshold from the CRC check result received from the CRC error detector 321.
Referring to FIG. 6, a closed-loop power control method in the above reverse link receiver will be described. In step 601, the SNR calculator 313 calculates an SNR from Nt and Eb measured by the noise measurer 311 and the bit energy measurer 309, respectively. Upon receipt of the SNR from the SNR calculator 313, the closed-loop power controller 315 compares the SNR with a fixed threshold in step 603. If the SNR is greater than the threshold, the closed-loop power controller 315 transmits a power-down command (PCB=0) to a mobile station in step 605. If the SNR is not greater than the threshold, the closed-loop power controller 315 transmits a power-up command (PCB=1) to the mobile station in step 607.
FIG. 4 is a block diagram of a forward link receiver in a DTX mode in the conventional CDMA mobile communication system. The structure and operation of the forward link receiver will be described referring to FIG. 4. In FIG. 4, a squarer 401 squares an input signal in sub-chip units. An accumulator 403 sums sub-chip energies for one Power Control Group (PCG). The sum is estimated as noise energy. A matching filter 405 filters the input signal in sub-chips units. A first despreader 407 PN-despreads the output of the matching filter 405 and outputs the PN-despread signal to a second despreader 409, a channel estimator 411, and a third despreader 413. The third despreader 413 despreads a pilot channel signal included in the PN-despread signal with a Walsh code. An accumulator 415 sums chip energies of the Walsh-spread signal. A squarer 417 squares the sum and outputs the square to an SNR calculator 417. The output of the squarer 417 is estimated as bit energy.
The other components are the same as their counterparts shown in FIG. 3 in structure but labeled with different reference numerals. The forward link receiver also performs a closed-loop power control in the same manner as shown in FIG. 6.
FIG. 5 illustrates DCCH transmission on a forward link and a reverse link in a DTX mode according to the IS-95C standard. The forward DCCH transmits data discontinuously and PCBs continuously regardless of the presence or absence of data. Also on the reverse link, data is discontinuously transmitted on the DCCH. If no data to be transmitted exists, pilot symbols and PCBs are transmitted on a pilot channel. Hence, the DCCH transmits no PCBs.
In the case of a traffic channel which continuously transmits frames, a receiver can perform an outer-loop power control continuously to obtain an intended frame error rate (FER). However, since the DCCH transmits in a DTX mode, the outer-loop power control can be used only when transmission frames are present.
FIG. 7 is a flowchart illustrating a general outer-loop power control method. The outer-loop power control method will be described with reference to FIGS. 3 and 7. Upon receipt of frame data, the outer-loop power controller 329 determines whether a frame error has been generated based on a CRC error check result received from the CRC error detector 321 in step 701. If a frame error exists, the outer-loop power controller 329 receives a frame existence flag from the data detector 323. If the frame existence flag indicates existence of a frame, the outer-loop power controller 323 increases a threshold in step 703. If the frame existence flag indicates the absence of a frame, the outer-loop power controller 323 decreases the threshold for power control in step 705. Procedures other than the above one can be employed for the outer-loop power control.
When the outer-loop power control method and the closed-loop power control method are used concurrently, a threshold updated for each frame in the outer-loop power control method is used as a reference SNR value in the closed-loop power control method.
FIG. 18 is a block diagram of a receiver for processing a DPCH (Dedicated Physical Channel) received in a DTX mode in an asynchronous IMT-2000 system employed in Japan and Europe. In FIG. 18, a channel separator 1805 separates a DPCCH (Dedicated Physical Control Channel) from an input DPCH. A channel estimator 1809 obtains information about channel status from the DPCCH received from the channel separator 1805, using pilot symbols. A multiplier 1806 multiplies DPCCH frame data received from the channel separator 1805 by the channel status information signal received from the channel estimator 1809. An SNR measurer 1807 calculates pilot energy Eb and noise energy Nt from pilot symbols. A bit energy measurer 1815 receives a DPDCH (Dedicated Physical Data Channel) and the multiplied DPCCH, compares their energies, and outputs the comparison result to a data detector 1819. The other components are described above with reference to FIG. 3. For implementation of an outer-loop power control and a closed-loop power control, the European IMT-2000 system is of the same structure and operates in the same manner, except for the above-described components.
As described above, the conventional outer-loop power control method is not applied when no frame exists on a DTX mode channel like a DCCH since an outer-loop power control is performed based on a determination whether a received frame has an error or not.
Therefore, if no frame is to be transmitted in the DTX mode, a threshold set for a previous frame is used. As a result, when frame transmission resumes and the previous threshold is higher than a threshold which should be set for receiving the current frame without errors, unnecessary transmission power is consumed. On the other hand, if the previous threshold is lower than the desired threshold, frame errors are increased. The increase of frame errors and transmission power dissipation decreases communication quality and base station capacity.
It is, therefore, an object of the present invention to provide a device and method for implementing an outer-loop power control in a DTX mode regardless of the presence or absence of data in a CDMA mobile communication system.
It is another object of the present invention to provide a device and method for implementing an outer-loop power control in a DTX mode regardless of the presence or absence of data by tabulating FERs versus SNRs and determining whether frame errors exist by referring to the table upon absence of transmission data in a CDMA mobile communication system.
It is a further object of the present invention to provide a device and method for implementing an outer-loop power control in a DTX mode regardless of the presence or absence of data by tabulating FERs versus data service types and determining whether frame errors exist by referring to the table upon absence of transmission data in a CDMA mobile communication system.
These and other objects can be achieved by providing an outer-loop power control device and method in a DTX mode in a CDMA mobile communication system. According to an embodiment of the present invention, in an outer-loop power control method for a non-frame data transmission period of a dedicated control channel (DCCH) which transmits frame data discontinuously, the signal-to-noise ratio (SNR) of power control bits (PCBs) received at a mobile station from a base station is measured over the frame period, and it is determined whether the frame has errors based on the measured SNR. A closed-loop power control threshold is increased if a frame error exists and decreased if no frame errors exist.
According to another aspect of the present invention, in an outer-loop power control device for a non-frame data transmission period of a DCCH which transmits frame data discontinuously, an SNR measurer measures the SNR of PCBs received at a mobile station from a base station for the frame period, a frame error detector determines whether the frame has an error based on the measured SNR and outputs a frame error indicator according to the determination, and an outer-loop power controller controls a closed-loop power control threshold according to the frame error indicator.